io_uring: provide FIFO ordering for task_work

task_work is a LIFO list, due to how it's implemented as a lockless
list. For long chains of task_work, this can be problematic as the
first entry added is the last one processed. Similarly, we'd waste
a lot of CPU cycles reversing this list.

Wrap the task_work so we have a single task_work entry per task per
ctx, and use that to run it in the right order.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -27,15 +27,6 @@ enum io_wq_cancel {
 	IO_WQ_CANCEL_NOTFOUND,	/* work not found */
 };
 
-struct io_wq_work_node {
-	struct io_wq_work_node *next;
-};
-
-struct io_wq_work_list {
-	struct io_wq_work_node *first;
-	struct io_wq_work_node *last;
-};
-
 static inline void wq_list_add_after(struct io_wq_work_node *node,
 				     struct io_wq_work_node *pos,
 				     struct io_wq_work_list *list)

fs/io_uring.c

@@ -721,6 +721,11 @@ struct async_poll {
 	struct io_poll_iocb	*double_poll;
 };
 
+struct io_task_work {
+	struct io_wq_work_node	node;
+	task_work_func_t	func;
+};
+
 /*
  * NOTE! Each of the iocb union members has the file pointer
  * as the first entry in their struct definition. So you can
@@ -779,7 +784,10 @@ struct io_kiocb {
 	 * 2. to track reqs with ->files (see io_op_def::file_table)
 	 */
 	struct list_head		inflight_entry;
-	struct callback_head		task_work;
+	union {
+		struct io_task_work	io_task_work;
+		struct callback_head	task_work;
+	};
 	/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
 	struct hlist_node		hash_node;
 	struct async_poll		*apoll;
@@ -2129,6 +2137,81 @@ static inline struct io_kiocb *io_req_find_next(struct io_kiocb *req)
 	return __io_req_find_next(req);
 }
 
+static bool __tctx_task_work(struct io_uring_task *tctx)
+{
+	struct io_wq_work_list list;
+	struct io_wq_work_node *node;
+
+	if (wq_list_empty(&tctx->task_list))
+		return false;
+
+	spin_lock(&tctx->task_lock);
+	list = tctx->task_list;
+	INIT_WQ_LIST(&tctx->task_list);
+	spin_unlock(&tctx->task_lock);
+
+	node = list.first;
+	while (node) {
+		struct io_wq_work_node *next = node->next;
+		struct io_kiocb *req;
+
+		req = container_of(node, struct io_kiocb, io_task_work.node);
+		req->task_work.func(&req->task_work);
+		node = next;
+	}
+
+	return list.first != NULL;
+}
+
+static void tctx_task_work(struct callback_head *cb)
+{
+	struct io_uring_task *tctx = container_of(cb, struct io_uring_task, task_work);
+
+	while (__tctx_task_work(tctx))
+		cond_resched();
+
+	clear_bit(0, &tctx->task_state);
+}
+
+static int io_task_work_add(struct task_struct *tsk, struct io_kiocb *req,
+			    enum task_work_notify_mode notify)
+{
+	struct io_uring_task *tctx = tsk->io_uring;
+	struct io_wq_work_node *node, *prev;
+	int ret;
+
+	WARN_ON_ONCE(!tctx);
+
+	spin_lock(&tctx->task_lock);
+	wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
+	spin_unlock(&tctx->task_lock);
+
+	/* task_work already pending, we're done */
+	if (test_bit(0, &tctx->task_state) ||
+	    test_and_set_bit(0, &tctx->task_state))
+		return 0;
+
+	if (!task_work_add(tsk, &tctx->task_work, notify))
+		return 0;
+
+	/*
+	 * Slow path - we failed, find and delete work. if the work is not
+	 * in the list, it got run and we're fine.
+	 */
+	ret = 0;
+	spin_lock(&tctx->task_lock);
+	wq_list_for_each(node, prev, &tctx->task_list) {
+		if (&req->io_task_work.node == node) {
+			wq_list_del(&tctx->task_list, node, prev);
+			ret = 1;
+			break;
+		}
+	}
+	spin_unlock(&tctx->task_lock);
+	clear_bit(0, &tctx->task_state);
+	return ret;
+}
+
 static int io_req_task_work_add(struct io_kiocb *req)
 {
 	struct task_struct *tsk = req->task;
@@ -2149,7 +2232,7 @@ static int io_req_task_work_add(struct io_kiocb *req)
 	if (!(ctx->flags & IORING_SETUP_SQPOLL))
 		notify = TWA_SIGNAL;
 
-	ret = task_work_add(tsk, &req->task_work, notify);
+	ret = io_task_work_add(tsk, req, notify);
 	if (!ret)
 		wake_up_process(tsk);
 
@@ -2157,7 +2240,7 @@ static int io_req_task_work_add(struct io_kiocb *req)
 }
 
 static void io_req_task_work_add_fallback(struct io_kiocb *req,
-					  void (*cb)(struct callback_head *))
+					  task_work_func_t cb)
 {
 	struct task_struct *tsk = io_wq_get_task(req->ctx->io_wq);
 
@@ -2216,7 +2299,7 @@ static void io_req_task_queue(struct io_kiocb *req)
 {
 	int ret;
 
-	init_task_work(&req->task_work, io_req_task_submit);
+	req->task_work.func = io_req_task_submit;
 	percpu_ref_get(&req->ctx->refs);
 
 	ret = io_req_task_work_add(req);
@@ -2347,7 +2430,7 @@ static void io_free_req_deferred(struct io_kiocb *req)
 {
 	int ret;
 
-	init_task_work(&req->task_work, io_put_req_deferred_cb);
+	req->task_work.func = io_put_req_deferred_cb;
 	ret = io_req_task_work_add(req);
 	if (unlikely(ret))
 		io_req_task_work_add_fallback(req, io_put_req_deferred_cb);
@@ -3392,7 +3475,7 @@ static int io_async_buf_func(struct wait_queue_entry *wait, unsigned mode,
 	req->rw.kiocb.ki_flags &= ~IOCB_WAITQ;
 	list_del_init(&wait->entry);
 
-	init_task_work(&req->task_work, io_req_task_submit);
+	req->task_work.func = io_req_task_submit;
 	percpu_ref_get(&req->ctx->refs);
 
 	/* submit ref gets dropped, acquire a new one */
@@ -5083,7 +5166,7 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
 	list_del_init(&poll->wait.entry);
 
 	req->result = mask;
-	init_task_work(&req->task_work, func);
+	req->task_work.func = func;
 	percpu_ref_get(&req->ctx->refs);
 
 	/*
@@ -8086,6 +8169,10 @@ static int io_uring_alloc_task_context(struct task_struct *task)
 	io_init_identity(&tctx->__identity);
 	tctx->identity = &tctx->__identity;
 	task->io_uring = tctx;
+	spin_lock_init(&tctx->task_lock);
+	INIT_WQ_LIST(&tctx->task_list);
+	tctx->task_state = 0;
+	init_task_work(&tctx->task_work, tctx_task_work);
 	return 0;
 }
 

include/linux/io_uring.h

@@ -22,6 +22,15 @@ struct io_identity {
 	refcount_t			count;
 };
 
+struct io_wq_work_node {
+	struct io_wq_work_node *next;
+};
+
+struct io_wq_work_list {
+	struct io_wq_work_node *first;
+	struct io_wq_work_node *last;
+};
+
 struct io_uring_task {
 	/* submission side */
 	struct xarray		xa;
@@ -32,6 +41,11 @@ struct io_uring_task {
 	struct io_identity	*identity;
 	atomic_t		in_idle;
 	bool			sqpoll;
+
+	spinlock_t		task_lock;
+	struct io_wq_work_list	task_list;
+	unsigned long		task_state;
+	struct callback_head	task_work;
 };
 
 #if defined(CONFIG_IO_URING)